The present invention is directed to a recording device, particularly an engraving device for a printing form engraving machine utilized to engrave printing plates or cylinders. The device comprises a shaft that is oscillated through a very small rotary angle, an engraving needle carried on a lever attached to one end of the shaft and a dampening arrangement connected to the shaft.
In a printing form engraving machine for engraving a printing plate or printing cylinders with a pattern to be printed which pattern may be writings, half-tone pictures or combinations of both, the pattern is scanned by a dot-like light in a line-by-line sequence. The light, which is reflected by the pattern, is measured for the intensity of the reflected light, which depends on the density of the scanned portion of the pattern, and is transformed into electrical picture signals. The picture signals are superimposed with a raster signal of a constant frequency for the raster pattern which is to be formed on the surface of the printing plate or cylinder.
The recording device of the printing form engraving machine utilizes an engraving system or device which is controlled by the combination picture and raster signal to actuate an engraving needle as a cutting tool. The raster signal causes a continuously vibrating lift movement of the engraving needle to form or cut a succession of depressions which are referred to as cups or cells on the surface of the printing form. The cups formed on the printing plate or cylinder will be in a raster pattern. The picture signal, which is an indication of the measured density of each particular portion of the pattern being reproduced, will determine the cutting depth of the engraving needle while forming each cup. Thus, during scanning of the pattern, a black part has a large density and will produce a picture signal that will cause the engraving needle to cut a deep cup. However, a white portion of the pattern will have little density and result in a picture signal which will control the engraving needle to produce a shallow cup.
In utilizing the engraved printing form during the printing process, each cup will contain a volume, which is proportional to the depth of the cup of printing ink, which may be a colored ink. The excess ink is wiped from the surface of the form by a doctor blade. The ink contained in the cup of the printing form is transferred from the cups onto the web on which the printing is to appear. The tone value of the printed surface at each point of the printed image is determined by the volume of ink carried by the cup at each point in the raster pattern.
During the engraving of each of the printing forms, the position of each of the cups with respect to other cups and their shape and depth have to be closely observed. Deviations in the arrangement of the cups with respect to other cups will lead to moire patterns and to color shift if several different colored inks are used.
In the printing process, there are approximately 120 tone value stages to distinguish between the density of "black" and "white" of the pattern. During the engraving, the needle lift difference for one tone value stage to the next adjacent tone value stage is approximately 0.25 .mu.m. Any deviation in needle lift changes the depth of the respective cup and thus the printed tone value.
Imperfections of the above type are sensed by the human eye as being disturbing when observing the printed picture. The above described requirements make great demands in respect to an engraving system or device which will be described in more detail in the following paragraphs.
The engraving device consists of a stationary electromagnet having an air gap which receives an armature mounted on a shaft of a rotary system. The coil of the electromagnet is energized by a control current which is proportional to the picture signal and raster signal. The rotary system also includes a bearing for supporting the shaft for rotation and dampening means. One end of the shaft is changed into a torsion rod which is spatially mounted from the other end which carries a lever on which the engraving needle is attached.
The magnetic field of the electromagnet electrically rotates the armature and the shaft, from a fixed resting position and the torsion rod assembly opposes the rotation by the electromagnet and applies a force restoring the shaft to the resting position. During each cup engraving step, the shaft is rotated through a very small rotary angle from the resting position which is determined by the non-twisted torsion rod and the angle of rotation is proportional to the amount or intensity of the control current which is determined by the picture signal and the raster signal.
Since the torsion rod assembly creates a spring-mass-system in case of abrupt changes of the control current, rotary inertia and additional attenuation due to dynamic moments will occur. The transit response of the engraving needle is determined by the degree of attenuation at its desired level which is determined by the control current.
In order to give the rotary system a desired degree of attenuation, a dampening arrangement is provided.
It is known from measuring techniques to dampen rotary systems by utilizing wings or pistons which move in a closed-off chamber containing a dampening medium. These arrangements have a common fact that the surface of the wings or the pistons which are effective for dampening are arranged perpendicular to the direction of movement of the rotary system. The known dampening arrangements cannot be used in the above engraving system since the degree of dampening which can be achieved thereby is insufficient due to the very small rotary angle of the shaft. Furthermore, it has been established that the characteristics of the dampening medium change due to mechanical stresses to the bordering surfaces of the dampening wings. This is due to the shaft and the dampening wings oscillating at a high frequency in the dampening medium.
After many years of preliminary work on solving the dampening problem, a dampening arrangement was developed for the engraving system or device wherein a single dampening wing, which is designed with a large surface area, was connected to the shaft at a point spaced from the lever and submerged into a stationary chamber which is filled with grease as a dampening medium. Unlike the known embodiments, the broad sides or surfaces of the dampening wing extend normal to the shaft and in the direction of rotation of the shaft. The degree of dampening of this proposed arrangement depends largely on the characteristics of the grease.
Even though the grease dampening device initially fulfills its task, after long periods of time, such as several years of practice, considerable shortcomings in respect to a mass production and maintenance of the engraving systems have become obvious. For example, the production of grease, as a dampening medium, with reproducible characteristics has not been achieved. Since the dampening characteristics of the grease change due to the influence of temperature and due to aging, the oscillation amplitude of the engraving needle would become unstable. Mistakes resulting from these defects will occur in the engraving operation and particularly during the scanning of patterns with large density differences at the transit points between dark and light portions in the pattern. These mistakes become disturbingly visible at these transit points when the entire pattern is observed.
If the engraving device has insufficient dampening, several contours may occur in the picture at the transit points from dark to light portions of the pattern. However, in the case of excessive dampening of the engraving device, the tone value determined by the pattern is produced in the picture with delay.
A further disadvantage of the described dampening arrangement is that the frictional forces between the dampening wing and the dampening medium are applied on one side of the shaft. This results in a bending moment, which fluctuates with the frequency of the oscillating system, and which moment tends to induce undesirable bending oscillations to the shaft. Since the resonant frequency of the bending oscillations is located in the proximity of the operating frequency of the oscillating system, surges will occur.
The deflection of the shaft with a surge frequency due to the bending oscillations has two main directions. One direction is in the direction of the lift movement of the engraving needle and superimposes the effective lift. This direction of bending oscillation in the case of large differences in density in the pattern causes several contours to appear in the finished picture. The second direction of deflection is perpendicular to the lift movement of the needle. Another deflection, perpendicular to the lift movement of the needle is achieved by forces applied to the lever during the insertion of the engraving needle into the material of the rotating printing form. The direction of deflection perpendicular to the lift movement of the engraving needle leads to the cutting of distorted cups which in the finally printed picture results in incorrect tone values. It has also been shown that the described dampening arrangements can lead to imperfections during the engraving of printing forms which can only be corrected by the production of a new form involving additional cost and time.